It is a design goal that when an aircraft wing is subjected to a momentary force in flight which causes the wing to oscillate between a bended state and an unbended state, that in the absence of this force, the oscillations will damp out and the wing will return to a steady, unbended state. On the other hand, "wing flutter" refers to a phenomenon in which the wing oscillations between the bended state and the unbended state do not damp out. Rather, the amplitude of these oscillations either remains constant or increases over time.
Wing flutter is an aeroelastic instability produced by the coalescing and proper phasing of two or more structural vibration modes of an aircraft in flight. A flutter mode usually involves both bending and torsion types of motion in which the torsional motion extracts energy from the airstream and drives the bending mode to increasingly higher amplitudes. In other cases these oscillations are lightly damped, but stable, within the operating speed envelope of the aircraft and can cause a reduction in the riding comfort of the aircraft.
The location of the engine nacelle relative to the wing, the mass properties of the engine, and the stiffness of the strut which attaches the nacelle to the wing are factors which influence the flutter characteristics of the wing. More specifically, the natural frequency of the nacelle and the manner of strut installation can influence the mode and airspeed at which the wing oscillations become unstable (flutter).
Conventionally, in order to avoid wing flutter, the natural frequency of the nacelles and nacelle struts are restricted within a narrow range. For example, in earlier models of the Boeing 747 aircraft, the outboard engine nacelles are permitted to oscillate at a natural frequency of about two cycles per second in a lateral direction. If the outboard engine nacelle lateral frequencies are significantly above or below two cycles per second then wing flutter can result at an unacceptably low airspeed.
However, in some newer aircraft which feature stronger but less stiff lifting surfaces, flutter can occur at airspeeds below that required by government regulations. In this case, the avoidance of wing flutter requires the unsatisfactory solution of reducing the maximum operating speed of the aircraft.
Other methods have been disclosed for preventing wing flutter, such as by adding damping materials, or by changing the relative positions of a component mass and/or center of pressure. For example, U.S. Pat. No. 2,124,098 by Younger pertains to an airfoil flutter damping device which includes an auxiliary airfoil device which attaches to a main airfoil to counteract flutter forces in the main airfoil.
Furthermore, U.S. Pat. No. 3,327,965 by Bockrath discusses a damping device which dissipates the oscillatory energy transmitted from an aircraft wing to an attached engine nacelle in order to prevent undesirable motion of the engine nacelle.
Other systems for preventing wing flutter include U.S. Pat. No. 3,734,432 by Low which pertains to a system for using leading and trailing edge control surfaces under the control of a stability augmentation system to damp out wing flutter.
A pneumatic spring system for suspending a store from a wing in order to reduce wing flutter is disclosed in U.S. Pat. No. 4,343,447 by Reed III.
And, U.S. Pat. No. 4,502,652 by Breitbach pertains to a spring device for suppressing wing flutter when carrying external loads.